An ink jet system (in particular, bubble jet system) disclosed in U.S. Pat. No. 4,723,129, U.S. Pat. No. 4,740,796 and so forth can provide high speed, high density and high definition recording of a high quality and is suitable for color recording and also for compact designing. Accordingly, progressively increasing attention has been paid to such ink jet system in recent years. In a representative one of apparatus which employ such system, ink (recording liquid or the like) is discharged making use of heat energy, and accordingly, it has a heat acting portion which causes heat to act upon the ink. In particular, a heat generating resistor having a heat acting portion is provided for an ink pathway, and making use of heat energy generated from the heat generating resistor, ink is heated suddenly to produce an air bubble by which the ink is discharged.
The heat acting portion has, from a point of view of causing heat to act upon an object, a portion apparently similar in construction to a conventional so-called thermal head. However, the heat acting portion is quite different in fundamental technology from a thermal head in such points that it contacts directly with ink, that it is subjected to a mechanical shock which is caused by cavitations produced by repetitions of production and extinction of bubbles of ink, or in some cases, further to erosion, that it is subjected to a rise and a drop of temperature over almost 1,000.degree. C. for a very short period of time of the order of 10.sup.-1 to 10 microseconds, and so forth. Accordingly, the thermal head technology cannot naturally be applied to the bubble jet technology as it is. In other words, the thermal head technology and ink jet technology cannot be argued on the same level.
By the way, as for a heat acting portion of an ink jet head, since it is subjected to such severe environment as described above, it is a common practice to employ such a structure that an electric insulating layer made of, for example, SiO.sub.2, SiC, Si.sub.3 N.sub.4 or the like is provided as a protective film on a heat generating resistor and a cavitation resisting layer made of Ta or the like is provided further on the electric insulating layer in order to protect the heat acting portion from environment in which it is used. As composing materials of such protective layer for use with an ink jet head, such materials which are tough against a shock and erosion by a cavitation as are described, for example, in U.S. Pat. No. 4,335,389 can be cited. It is to be noted that an abrasion resisting layer made of Ta.sub.2 O.sub.5 or the like popularly used for a thermal head is not always superior in cavitation resisting property.
Apart from this, it is desired for a heat acting portion of an ink jet head to be constituted such that heat generated from a heat generating resistor acts upon ink as efficiently and quickly as possible in order to save power consumption and improve the responsibility to an input signal. To this end, apart from the aforementioned form in which a protective layer is provided, also a form in which a heat generating resistor contacts directly with ink is proposed in Japanese Patent Laid-Open No. 126462/1980.
A head of the form is superior with regard to thermal efficiency to the form in which a protective layer is provided. However, not only is a heat generating resistor subjected to a shock or erosion by a cavitation and further to a rise and a drop of temperature, but also it is subjected to an electrochemical reaction which is caused by electric current which flows through recording liquid because the recording liquid which contacts with the heat generating resistor has an electric conductivity. Consequently, various metals, alloys, metallic compounds or cermets beginning with Ta.sub.2 N and RuO.sub.2 which are conventionally known as materials of heat generating resistors are not always satisfactory in durability or stability for an application to a heat generating resistor of a head of the form.
While some of ink jet heads of the form wherein a protective layer is provided as described above which have been proposed so far can be adopted in practical use as regards durability and resistance variation, it is very difficult, in any case, to perfectly prevent occurrence of defects which may take place upon formation of a protective layer, which is a serious factor of deteriorating the yield in mass production. Then, further improvement in speed and density in recording is demanded, and since there is a tendency that the number of discharging outlets of a head is increased corresponding to such demand, this is a serious problem.
Further, while a protective layer described above decreases the efficiency in transfer of heat from a heat generating resistor to recording liquid, if the heat transfer efficiency is low, then the entire power consumption required increases and the temperature variation of the head upon driving increases. Such temperature variation results in volume variation of a droplet discharged from a discharging outlet, which causes a variation in density of an image. Meanwhile, if the number of discharging operations per unit time is increased in order to cope with an increase in recording speed, the power consumption by the head is increased accordingly and the temperature variation is increased. Such temperature variation will bring about a corresponding density variation of an image obtained. Also when an increase in number of discharging outlets which involves an increases in density of electrothermal converting bodies, the power consumption by the head increases, and a temperature variation by such increase in power consumption will likewise cause an image obtained to have a density variation corresponding to such temperature variation. Such problem that an image obtained has a density variation is contrary to a demand for a high quality of a recorded image and is required to be solved as early as possible.
In order to solve such problem, provision is desired earnestly of an ink jet head wherein a heat generating resistor contacts directly with ink and the heat efficiency is high.
However, since a heat generating resistor of an ink jet head of the conventional form wherein ink contacts directly with the heat generating resistor is subjected not only to a shock or erosion by a cavitation and further to a rise and a drop of temperature but also to an electrochemical reaction as described hereinabove, conventional materials for a heat generating resistor such as Ta.sub.2 N, RuO.sub.2 or HfB.sub.2 have a problem in durability in that the heat generating resistor may be mechanically destroyed, or corroded or dissolved.
The materials which are disclosed as tough against a shock or erosion by a cavitation in U.S. Pat. No. 4,335,389 and so forth do not exhibit their effects if they are not used for such a protective layer (cavitation resisting layer) as described hereinabove. However, if any of the materials is employed for a heat generating resistor which contacts directly with ink, then it is sometimes dissolved or corroded by an electrochemical reaction, and consequently, it may assure a sufficient durability.
Further, the stability of discharging is inevitable for recording of a high definition and a high quality, and to this end, it is necessary that the resistance variation of a heat generating resistor be low, and for practical use, preferably it is lower than 5%. Ta or Ta-Al alloy mentioned in Japanese Patent Laid-Open No. 96971/1984 is comparatively superior, where it is employed for a heat generating resistor of an ink jet head which contacts directly with ink, in durability, that is, in cavitation resisting property in that the resistor is not broken. However, with regard to a resistor variation during a repetition of production of bubbles, Ta or a Ta-Al alloy is not satisfactory in that the resistor variation is not very small. Further, Ta or a Ta-Al alloy does not have a very high ratio M between an applied pulse voltage (V.sub.break) at which the resistor is broken and a bubble producing threshold voltage (V.sub.th) and is not very high in heat resisting property, and consequently, they have a problem that the life of the resistor is deteriorated significantly by a small increase of a driving voltage (V.sub.op) In particular, Ta or a Ta-Al alloy is not always sufficiently high in resisting property to an electrochemical reaction, and consequently, where it is employed as a material for a heat generating resistor for an ink jet head which contacts directly with ink, if production of bubbles is repeated by a large number of application pulses, then the electric resistance of the heat generating resistor is varied to a great extent. Thus, there is a problem that also the condition of production of bubbles is varied by such variation of the electric resistance of the heat generating resistor. Further, there is another problem that, since the heat resisting property is not very high, a small variation of V.sub.op sometimes has a significant influence on the life of the resistor.
In this manner, even if a heat generating resistor which contacts with recording liquid (that is, ink) is formed from any of the conventionally known materials, an ink jet head or an ink jet apparatus cannot be obtained readily which can satisfy all of a cavitation resisting property, erosion resisting property, mechanical durability, chemical stability, electrochemical stability, resistance stability, heat resisting property, oxidation resisting property, dissolution resisting property and thermal shock resisting property.
Particularly, an ink jet head or an ink jet apparatus cannot be obtained readily which has a structure wherein a heat generating resistor is provided for direct contact with ink and is high in heat transfer efficiency, superior in signal responsibility and sufficiently high in durability and discharging stability.